This invention relates, in general, to the measurement of particulate matter suspended in a fluid medium and, more specifically, to measurement of the mass and/or concentration of particulate matter suspended in ambient air or in other gaseous environments, e.g. in diesel exhaust, or in mines, smoke stacks, industrial facilities, etc. Particulate matter is the general term which refers to condensed solid, semi-solid, or liquid material produced as a result of natural or man-made processes, and which due to small size, is capable of being suspended in the air or other fluid medium.
The measurement of particulate matter in ambient air is important for a variety of reasons, the most important of which is related to health effects. Suspended particulate matter is known to produce a variety of deleterious health effects when inhaled. As a result, regulatory agencies around the world require monitoring of the levels of particulate matter. The levels are measured in terms of concentration, i.e. micrograms of particulate matter per cubic meter of air. Reference techniques for this measurement are presently defined in terms of a mass measurement utilizing a filter medium to capture the particulate matter and the total volume of air which has been filtered by the medium over a given period of time. There are various means available to unambiguously determine the flow rate through the filter over time (and hence the volume of air sampled), but surprisingly the mass measurement is not straightforward due to the complex nature of ambient particulate matter which results in unstable mass deposition on the filter.
This problem involving the measurement of particulate matter in ambient air is well-known. The uncertainty arises since the particulate mass used as a basis for mass concentration computations is defined as the mass captured on the filter media which is not necessarily the mass of the particles as they exist in the ambient air. Unlike measurements of major criteria gaseous pollutants, what is defined as particulate matter can change its mass as a result of loss or gain of volatile substances associated with the particulate matter and filter media. While gaseous pollutants exist as definable molecular species (SO2, O3, CO, etc.), particulate matter can be a combination of different substances with different volatilization rates, reactive, desorptive, absorptive, and adsorptive properties. In addition, the mass of particulate matter on the filter can be affected by the filter material itself, the particulate matter already collected on the filter, the face velocity through and pressure drop across the filter, as well as by the humidity, temperature and composition of the gas stream passing through the collection medium.
Both direct and indirect measurement techniques have been employed in an effort to quantify particulate matter mass. Each method which has been developed to date, however, has limitations in obtaining a measurement of the actual mass of particulate matter as it exists in its suspended form. Direct mass measurements as represented by weighing material captured on a substrate such as a filter are susceptible to instrument effects due, for example, to temperature or pressure changes, and to volatile component losses which are not easily quantifiable. Indirect methods such as light scattering measurements on the other hand are inherently inaccurate as there is no physical connection between other properties of particles and particle mass.
To compensate for instrument effects in direct mass measurements, a differential particulate mass measurement microbalance employing a pair of oscillating quartz crystal detectors has previously been proposed. In this earlier approach, a particle laden gas stream impacts upon the first detector and a particle free gas stream impacts the second detector. The second mass detector is used as a reference to cancel out detector instrument effects from a mass reading provided by the first detector. U.S. Pat. No. 5,571,945 discloses a similar measurement approach employing a pressure sensor to measure a pressure differential between a pair of particulate matter collectors; U.S. Pat. No. 5,349,844 discloses a similar approach for use with a filter that is caused to oscillate in a direction substantially perpendicular to a plane of the filter. However, volatilization losses are not accounted for in these earlier systems.
As a result of the above described difficulties, the current reference method in the United States is a method dependent technique which does not necessarily represent an accurate measure of particulate mass as it actually exists in its undisturbed state in the air. The reference method consists of filter equilibration under a defined range of temperature and humidity conditions, a pre-collection weighing of the filter, the installation of the filter in a manual sampler, the sampling of ambient air (for a 24-hour period), the removal of the filter from the sampling device, a post-collection conditioning under the same equilibration conditions as before, and finally a post-collection weighing. This methodology is intended to provide a consistent set of measurements between identical samples.
However, for the reasons stated above, results based on this method do not represent measurements to which an accuracy can be assigned, even loosely, i.e. to what accuracy is the particulate mass as it exists in the atmosphere measured by the mass determined from the filter? This is a serious problem, and one has to accept the fact that these measurements are only an indication of particulate levels. As a result, the current reference method represents simply a standardized procedure, and not a scientifically-based measurement standard for airborne particulate matter.
Volatile components are a confounding influence on these measurements. While the filter resides in the sampling hardware, important factors that influence the reactions taking place on the filter substrate, such as temperature and humidity, vary in an ill-defined manner. During sampling, the mass on and of the filter can increase dramatically during periods of decreasing temperature and increasing relative humidity (nighttime), and may experience substantial loss of semi-volatile materials when the temperature increases and humidity decreases (daytime). These same type of effects can be associated with air mass changes, and other meteorological events. Further, the collection filter may be exposed to widely varying hot or cold temperatures once sampling is complete and before it is removed from the sampler as well as during transportation to a laboratory for conditioning and weighing.
Not only does the mass of collected particulate matter and the filter change depending upon the conditions to which they are exposed, but the air stream through the filter creates a pressure differential across the filter which tends to strip off volatile components of the particulate matter. In effect, the interaction of the particles with the filter tends to modify the nature of the particulate matter as soon as it is collected, thereby affecting the accuracy of the desired measurement of the particulate matter as it is suspended in ambient air. As health concerns heighten, and measurement instrumentation becomes more sensitive, there is a trend towards measurement of even finer particulate matter, e.g. particles of 2.5 microns or less. With smaller particles, the impact of volatilization losses upon the mass measurement readings becomes even more pronounced.
A compelling need thus exists for a measurement instrument that can accurately measure the mass or concentration of particulate matter suspended in ambient air or other gaseous environments.
The present invention provides a method and apparatus which overcomes the problems described above and provides a collection-based direct mass measurement that allows the accurate quantification of the mass of ambient air or other gas borne particulate matter including volatile components thereof. The measurement approach of the present invention not only cancels out detector instrument effects but also intrinsically corrects for volatilization losses. For purposes of this disclosure, the term xe2x80x9cvolatilization lossesxe2x80x9d is used broadly to include vaporization, absorption, adsorption, desorption, reactive and other effects which influence (positively or negatively) the mass of collected particulate matter. Some of the effects are generally known as collector (e.g. filter) artifacts.
In accordance with the principles of the present invention, apparatus for measuring the mass of particulate matter in a particle laden gas stream includes a mass detector, and first means for providing a particle free gas stream otherwise substantially identical to the particle laden gas stream. Switching means causes said particle laden gas stream and said particle free gas stream to alternately engage said mass detector during successive measurement time periods. A difference between a reading provided by the mass detector for a current measurement time period and a reading provided by the mass detector for a consecutive measurement time period is computed. This difference intrinsically corrects for volatilization losses occurring during the current measurement time period. A measure of the mass or concentration of particulate matter in the particle laden gas stream is determined from this difference.
Advantageously, the first means for providing a particle free gas stream comprises particle removal means for removing substantially all particulate matter from said particle laden gas stream. Optimally, said particle removal means removes the particulate matter from the particle laden gas stream without appreciably affecting gas stream temperature, pressure and flow rate. Such particle removal is preferably accomplished using an electrostatic precipitator. The precipitator preferably operates with a positive corona and low current.
The mass detector may comprise an oscillating element microbalance. The detector may comprise a hollow element oscillating in a clamped-free mode, with a filter supported at a free end of the element. The filter serves to collect particulate matter from the particle laden gas stream when this stream engages the detector. Fluid control means can advantageously maintain a substantially constant gas stream flow at the filter of the detector during each measurement period. In the oscillating element microbalance embodiment, mass readings provided by the mass detector are advantageously based upon detected change of frequency of oscillation of the oscillating element with respect to time.
In another aspect of the invention, the switching means of the mass measuring apparatus causes: (a) the particle laden gas stream to engage the mass detector during each of odd numbered ones of the successive measurement time periods, and (b) said particle free gas stream to engage the mass detector during each of even numbered ones of the successive measurement time periods. When the detector is engaged by the particle laden gas stream, it measures mass gain; when the detector is engaged by the particle free gas stream, it measures mass lost due to volatilization of volatile components of the particulate matter. The measured mass lost is added to the measured mass gain to determine the measure of the mass of the particulate matter. Each successive measurement time period lasts for a short time, preferably fifteen minutes or less; about a minute or less being presently considered as most preferred.
In another aspect of the invention, the readings provided by the mass detector each comprise a mass rate reading, which limits accumulation of any calibration errors in the mass measurement.
In yet another aspect of the present invention, corrected mass concentration is computed from a corrected mass rate which combines mass rate readings from the mass detector for two successive measurement periods.
The present invention also presents a significant improvement to existing differential particle mass measurement systems. In such systems, a particle laden gas stream engages a first mass detector and a particle free gas stream engages a second mass detector. The second mass detector is used as a reference to cancel out detector instrument effects from a reading provided by the first mass detector. According to the present invention, a differential particle mass measurement system is improved by inclusion of switching means for causing the particle laden gas stream and the particle free gas stream to alternately engage a single mass detector, during successive measurement time periods. In this fashion, correction is intrinsically provided for volatilization losses occurring during the successive measurement time periods, calibration or matching of multiple detectors is avoided, and the complexity and cost of the measurement device is reduced.
In a further aspect of the present invention, apparatus for measuring the mass of particulate matter, including volatile components thereof, in a particle laden gas stream is provided. This apparatus includes a mass detector, means for directing the stream to continually engage the mass detector, and particle removal means for removing substantially all particulate matter from the stream when the particle removal means is activated. Control means activates the particle removal means for alternate successive measurement time periods. A difference is determined between a first reading provided by the mass detector and a second reading provided by the mass detector for successive measurement time periods. This difference intrinsically corrects for volatilization losses occurring during the measurement time periods. A measure of the mass or concentration of particulate matter in the particle laden gas stream is determined from this difference.
Pursuant to a still further aspect of the present invention, a differential particle mass measurement method is improved. In the known method, a particle laden gas stream engages a first mass detector. The first mass detector collects a current particle sample from the gas stream during a current measurement time period and measures mass gain due thereto. A second mass detector is used as a reference to cancel out detector instrument effects. The present invention improves upon this method by using a single mass detector to not only cancel out detector instrument effects but to also effectively measure a change in particle property occurring during said current measurement time period. This change in particle property usually comprises a loss of mass due to volatilization of collected volatile particles. The mass lost due to volatilization as measured by the mass detector is added to the mass gain measured by the mass detector to yield a corrected particle mass measurement for the current measurement time period. The measured loss of mass occurs during a consecutive measurement time period (i.e., during a period just before or after the current measurement time period) in an earlier collected particle sample; this earlier collected particle sample having been collected by the mass detector in a preceding measurement time period. Preferably, the current measurement time period and the consecutive measurement time period are of such short duration as to ensure substantially identical volatilization during said consecutive measurement time period of the earlier collected sample and the current particle sample.
The present invention provides numerous significant benefits and advantages. Foremost among these is its intrinsic correction for volatilization losses occurring during measurement time periods. Using short measurement time periods ensures that the particulate mass measurement includes an accurate representation of the volatile mass associated with the collected particulate under any selected temperature including varying ambient temperature conditions. Since the mass detector sees substantially identical collector (e.g. filter) and instrument artifacts during a pair of successive measurement time periods, compensation for instrument effects is effective and complete when the readings from one measurement time period are subtracted from those of the other. The preferred use of an electrostatic precipitator for particle removal prevents any pressure disturbance from occurring. The electrostatic precipitator also facilitates switching of the particle content of the gas stream on and off electrically and instantaneously with no mechanical motion being necessary. The switching of the gas stream also effectively expands collector (e.g. filter) life by a factor of 2 compared to a continuous particle collector system. Further, if two differential measurement instruments are run side-by-side in accordance with the principles of the present invention, one at ambient temperature and the other at a significantly elevated temperature, then a division between volatile and non-volatile components of the ambient particulate matter can be obtained. The use of a single detector provides additional cost and reliability benefits.